This invention relates generally to projection printing and more specifically to a projection printing system having a pattern mask structure which avoids the printing of contamination particles.
In the manufacture of integrated circuits, the various devices and circuit connections are formed using photoresist masks to protect the semiconductor substrate except in the areas where processing, such as etching or metal deposition, is desired to occur.
The photoresist masks are formed by coating the substrate with a layer of resist and then patternwise exposing the resist by passing ultraviolet light through the apertures of a pattern mask. The light causes the resist layer in the light struck areas to either harden, in the case of a negative resist, or to degrade, in the case of a positive resist. The unhardened or degraded areas are then removed by a developer to expose portions of the substrate for processing. The pattern masks are usually formed of a transparent substrate with a pattern of opaque areas of, for example, silver or chromium formed on one surface.
In the past, three types of resist printing have been used. The most common method is contact printing in which the patterned surface of the mask is pressed into contact with the resist layer. This has the disadvantage of limited mask life due to dirt pickup and physical damage to the mask resulting from the contact between the mask and substrate. The image size is also limited to a 1 to 1 reproduction. Proximity printing has been used to reduce contact damage. The image size is still limited to a 1 to 1 reproduction and a new problem is introduced by the out of contact exposure because of light diffraction effects which can reduce resolution and cause ghost images. More recently, projection printing using high quality optics has been employed. The separation of mask and substrate not only prolongs mask life, as in the case of proximity printing, but provides the ability to form not only 1 to 1 reproductions but reduced size images on the substrate. The mask image can then be, for example, 2 to 10 times the size of the image formed on the substrate. This makes the formation and checking of suitable masks easier because the images are larger. It also has the advantage that errors in the mask are reduced in size in the image formed on the substrate so that mask errors which would be harmful in a 1 to 1 magnification ratio system can be ignored. One disadvantage which has been found with projection printing is its sensitivity to particles of contamination which collect on the transparent areas of the patterned surface of the mask. Because the system is designed to bring the mask pattern image into sharp focus on the photoresist layer surface, the contamination particles are also brought into sharp focus with optimum resolution and contrast at the substrate plane so that the images of the larger particles are printed in the resist layer. This problem is compounded with systems which reduce the size of the image because of the much larger relative size of the mask area. This greatly increases the probability of contamination particles of a size large enough to print being located on a transparent area of the mask pattern even in a clean room environment.
Copending application Ser. No. 713,948 filed Aug. 12, 1976 entitled "Projection Printing System With An Improved Mask Configuration", now U.S. Pat. No. 4,063,812, discloses a solution to the dirt problem by encasing a mask in a glass sandwich so that any dirt particles on the surfaces of the sandwich are out of focus and are not printed.
We have now found an improved structure for the avoiding of the printing of dirt particles which is easy to construct and which has a minimal effect on the optical path of the projection printing system so that little if any adjustment is needed of the mask or image planes even with large separations between the mask pattern and the surface of the cover.